The invention relates to a method for dispensing a microgram or milligram sample of determined quantity from a powder or paste stored in a magazine through an outlet of that magazine. Furthermore, the invention relates to a device for the carrying out of such a method.
Solid samples in the microgram and the low milligram range are necessary, for instance, for the tracer analysis of food by means of an atomic absorption spectrometer. Pulverized solid samples of such a small volume do not behave like samples in the gram range, for which, for example in the pharmaceutical industry, metering devices exist which work satisfactorily. The differences in the physical behaviour of smaller samples are, first and foremost, a result of electrostatic charge, size and distribution of the particles, adhesion effects and humidity of the samples. All these features are very important for the metering of minimum quantities and make the known methods unsuitable.
DE-PS 32 04 873 already discloses a device for the automatic loading of a graphite tube furnace of an atomic absorption spectrometer with a sample. The sample material is metered out of a magazine by shaking or vibrating onto a sample carrier, the magazine having an outlet at its base. In practice, sample material of different samples is of different constitution. Therefore it is impossible to dispense sample material quantities by shaking or vibrating, which are sufficiently constant in their weight. Due to the physical behavior of the sample material the other metering devices known in chemical engineering do not render it possible to dispense automatically the extremely small sample quantities.
Due to these shortcomings of the device according to the mentioned German patent 32 04 873, in practice the samples, the quanity of which is determined by eye, are manually taken from a magazine on the sample carriers by means of a spatula, the quantity of which is determined by eye. This has the disadvantage that, apart from the fact that an automatic operation is impossible, sample material can be carried over by means of the spatula from one sample material magazine to the other.
The same risk exists with the metering device according to DE-PS 29 45 646. By means of this device a determined volume can be dispensed, but there is the risk that a part of the metered material falls from the device when transported to the sample carrier and that a small quantity adheres to the device instead of reaching the sample carrier. Furthermore, in consequence of different densities, the masses can be different in spite of same volumes.